Microbridge mass airflow sensors are manufactured using MEMS (Micro-Electro-Mechanical Systems) technology. The microstructure chip comprises of a thin-film, thermally isolated bridge structure containing a heater and temperature sensing elements, all spanning an etched cavity. Heater temperature is typically controlled to be several degrees above ambient temperature. Thermal flow sensors operate on the principles of heat transfer across the surface of the sensing element. The upstream sense resistors are cooled, the downstream sense resistors are heated, and the combined differential electrical signal is proportional to flow.
Microbridge sensors can be designed into safety critical applications. Sometimes in these applications the microbridge sensor can be utilized in a dynamic feedback loop and can be the only indication of flow in the entire system. In such instances, accuracy, repeatability and long term drift performance are all crucial factors. In safety critical applications, redundant sensors can be utilized and the outputs of each of the sensors may be averaged to minimize the effects of long term drift on accuracy performance. This can be an expensive solution and is also not completely fail safe.
If the output of the microbridge sensor can be measured at a known flow condition, then the end user can recalibrate (either electronically or via software) the output of the sensor and account for sensor drift. However, the most optimum condition to measure is zero flow. If the flow can be completely shut off in the system and the no flow null of the microbridge measures as direct measurement that can be made of long term drift. Often times it is either physically not possible or too expensive to shut off flow to the sensor or to put in a known flow condition.
One known trick that is performed and known in public domain is to shut of the heater to the microbridge, wait some amount of time that is predetermined, and then measure the bridge null. Usually the heater is turned on and off by the end user. A lot of thermal energy can be removed from the system when the heater is turned off.
In known prior art methods, attempt to measure bridge null drift makes only the microbridge heater to shut off. The action of reducing the supply voltage across the bridge and measuring the microbridge null only occurs when actively performed by the end user. The microbridge null will also be temperature compensated by the electronics within the sensor. When the heater is shut off and the microbridge null is measured, the value can change significantly depending on the specific temperature at which the measurement was taken.
Referring to FIG. 1, labeled as “prior art”, a cross-sectional view of a Microbridge mass airflow sensor, which can be used for and benefit from features of the present invention is illustrated. The body of the sensor 100 is a semiconductor, preferably silicon substrate 101 with an etched cavity 102, chosen because of its adaptability to prediction etching techniques and ease of electronic chip productivity. The airflow can be from a direction indicated by an arrow 103. The upstream sense resistors 104 can be cooled by the transportation of heat away from the resistors 104 toward the heater 105. Transportation of heat toward the downstream sense resistors 106 from the heater 105 heats the downstream sense resistor 106. A resistance difference between the upstream sense resistors 104 and downstream sense resistors 106 can be present with a corresponding difference in voltage drop. The difference in voltage drop can be the measure of airflow.
Based on the foregoing, it is believed that a need exists for a microbridge sensor design that overcomes such problems. It is believed that the system and method disclosed herein offers a solution to these problems by providing an improved Airflow Combi-sensor ASIC (Heidmal) design which includes a self diagnostic method that automatically occurs at start up whereby the temperature compensated microbridge null drift and performance can be measured.